1. Field of the Invention
This invention relates to a switch for controlling the transmission of light from a first optical fiber to at least a second optical fiber or for controlling the bidirectional flow of light in a single fiber.
2. Description of the Prior Art
Local area networks and industrial control systems using fiber optic cable as the medium for transmission of information and control signals are displacing systems that use metallic wires as the transmission medium. In fabricating such systems the ability to switch light from one fiber optic cable to another is important for system operation.
One class of switches for optical fibers disposes at least a first and a second optical fiber in generally confronting end-to-end, axially alignable relationship. In this class of switch one of the optical fibers is mounted on a fixed member while the other optical fiber is mounted on a movable member. An actuator is used to displace the movable member to bring the fibers into or out of the aligned relationship. Exemplary of such a switch is that disclosed and claimed in copending application Ser. No. 07/262,935. Other examples of this class of switches are those described in U.S. Pat. Nos. 4,416,856 and Re. 31,579 (both to Jaeschke) and U.S. Pat. No. 4,452,057 (Winzer). Each of the switches disclosed in these patents includes magnetically actuated switching elements. U.S. Pat. No. 4,303,302 (Ramsey et al.) discloses a piezoelectric actuated switching element. U.S. Pat. No. 4,223,978 (Klummer et al.) shows a motor-actuated, rotatably driven switching element.
There is also know in the art another class of switches in which the end faces of the optical fibers are nonconfrontationally disposed. In such an arrangement light emanating from one of the optical fibers is directed toward a reflector. The reflector acts to re-direct the light toward the end face of the other of the optical fibers. The reflector may take various forms, such as a corner reflector or a spherical reflector. The switch, in either form, may include one or more lenses of various types.
Exemplary of a switch employing corner reflectors is European Patent Application 12,274 (Aoyama). This application discloses, in one embodiment thereof, the use of a single corner reflector to direct the light from a first optical fiber to a second optical fiber. In a second embodiment a laterally displaceable prism is used to couple light from a first corner reflector to a second corner reflection and then to a third fiber. Another example of a switch employing a corner reflector is U.S. Pat. No. 4,607,160 (Sakakino), in which a corner reflector directs light from a first optical fiber to a second optical fiber. A moving vane is disposed in a slot between the two reflecting surfaces of the corner reflector to interrupt the light beam. None of the aforementioned patents is believed readily able to be used with a single bidirectional optical fiber and are believed disadvantageous in this regard.
Exemplary of a switch employing a spherical reflector is European Patent Application 299,604 (Kokoshvili), in which the spherical reflector is displaced laterally to effect the switching action. Another example of a switch using a spherical reflector is U.K. Application 2,014,751 (Merz and Hodge), in which a spherical reflector directs light from a first to a second optical fiber or from a first to a third fiber. In the device show in this application the spherical reflector is displaced rotationally to effect the switching action. The description of switches in these patents relates only to the use of physically separated optical fibers.
In industrial control applications it is especially important to verify system integrity. This may be accomplished in the case of an electrical industrial control system by connecting a high value resistor in parallel to a switch to provide a small loop current when the switch is open. In such a system the receiving circuit associated with the switch senses this current to verify loop integrity. If the loop current drops below a threshold level, a fault indication is produced which may be used to activate safety devices. None of the above mentioned patent references disclosing fiber optic systems provides an arrangement for detecting that the optical circuit is intact. For this reason the switches shown in the above mentioned patents are believed disadvantageous for industrial control applications.
In view of the foregoing it is believed advantageous to provide a switch that is usable with one bidirectional optical fiber or with one or more pairs of either single mode or multimode optical fibers. It is also believed advantageous to provide a switch that may be used in conjunction with appropriate functional elements to provide the capability to verify system integrity when using a fiber optic transmission medium. In particular, it is believed advantageous to detect that the optical circuit is intact when the switch is "off".